The recently discovered gastric antrum mucosal cell protein (AMP-18, now also designated gastrokine-1, Gkn-1) has marked physiological effects on simple epithelial cells in culture. These include mitogenic, motogenic and maturational activities. Quiescent epithelial cells are stimulated to limited mitogenesis, similar to (but additive with) that caused by epidermal growth factor (EGF). Scrape-wounded cultures are stimulated to repair wounds. AMP-18 and related synthetic peptides have been shown to accelerate the formation of tight junctions in confluent cultures of several cell lines capable of epithelial maturation in vitro. Further, cultures that have formed tight junctions can be protected against damaging agents by prior exposure to AMP-18 or related peptides. Since it appears that AMP-18 is cosecreted with mucins into the gastric lumen, those AMP-18 functions in a positive feedback loop to maintain the extent and functional integrity of the surface epithelial cell layer. The mechanisms which underlie these effects are unknown. The fact that AMP-18 can act on non-gastric epithelial cells raises number of significant issues: is there a family sharing a common ligand, or is there a general promiscuous receptor? The characterization of the cell receptors cells will give valuable insight into the underlying signalling mechanisms, and also the potentially significant interaction with other factors regulating growth and maturation of epithelia. AMP-agonist and - antagonist peptides greatly facilitate the physiological, and potentially pharmacological, role of the factor. Specifically: 1. The bioactive domain of AMP-18 will be explored by synthesis of high-affinity peptide analogues to serve as ligands; feasibility of determining an X-ray structure for the protein will be assessed. 2 the cellular receptors for AMP-18 will be identified by a combination of affinity procedures; if the receptors are novel proteins, they will be cloned, and antibodies produced to determine their tissue distribution and developmental expression, particularly in relationship to known growth and differentiation factors.